The present invention relates to a process for producing a biaxially oriented paraphenylene sulfide block copolymer film having a high Young's modulus. More particularly, the present invention relates to a process for producing a biaxially oriented paraphenylene sulfide block copolymer film having a high Young's modulus, comprising melting a paraphenylene sulfide block copolymer essentially composed of recurring units (A): ##STR4## and recurring units (B): ##STR5## to form a film and biaxially stretching the thus obtained film at a stretching temperature (T) defined by the formula: EQU 96-50X&lt;T&lt;116-66.7X
(wherein T is stretching temperature (.degree.C.) and X is molar fraction of recurring units (B)).
Paraphenylene sulfide polymer is known as a thermoplastic resin having high heat-resistance and high chemical-resistance and excellent electrical properties since the paraphenylene sulfide polymer can be used at its working temperature as high as nearly the crystalline melting point (about 285.degree. C.) by crystallizing it to a high degree because of its high crystallinity (see, for instance, Japanese Patent Publication No. 52-12240 (1977), Japanese Patent Publication No. 45-3368 (1970), Japanese Patent Application Laying Open (KOKAI) No. 59-22926 (1984) and U.S. Pat. No. 3,869,434). Also, some films made of such paraphenylene sulfide polymers and the processes for producing such films have been proposed.
For instance, there have been proposed a biaxially oriented poly-p-phenylene sulfide film containing more than 90% by mole of recurring units: ##STR6## and having a density of 1.330-1.400 g/cc at 25.degree. C., produced by melt extruding and molding a poly-p-phenylene sulfide having a melt viscosity in the range of 100-600,000 poises as measured at 300.degree. C. and shear rate of 200 sec.sup.-1 to form a non-crystalline (amorphous) transparent film, biaxially stretching the thus obtained film simultaneously or successively at 80.degree.-120.degree. C., and heat-setting the thus stretched film under tension at a temperature in the range from 180.degree. C. to the melting point of the polymer (Japanese Patent Publication No. 59-5100 (1984)); a biaxially oriented poly-p-phenylene sulfide film having a film-to-film kinematic friction coefficient of greater than 0.75 at 20.degree. C. and 70% RH and a film surface roughness of less than 0.9.mu./5 mm on the average, obtained by producing a poly-p-phenylene sulfide containing not less than 90% by mole of recurring units: ##STR7## by polymerization, melt extruding and molding this polymer to form a non-crystalline (amorphous) film, biaxially stretching this film at a temperature of 80.degree.-100.degree. C. and heat-setting the thus biaxially stretched film at a temperature of 150.degree.-280.degree. C., wherein the particles of an inert inorganic material such as silica, alumina, carbon, glass, calcium carbonate, calcium phosphate or the like are added during or at the end of the polymerization, a determined amount of insoluble salt used in the polymerization is left, or the film is treated by a surface roughening roll in the film forming process or subjected to surface oxidation treatment or blast finishing with a solid matter (Japanese Patent Application Laying Open (KOKAI) No. 55-34968 (1980)); a process for producing a poly-p-phenylene sulfide film comprising melt extruding and molding poly-p-phenylene sulfide containing not less than 90% by mole of recurring units: ##STR8## to form a substantially amorphous film, stretching this film by 2.0-5.0 times in one direction at 80.degree.-120.degree. C. to make the birefrigence index of the film 0.05-0.30, further stretching the thus obtained film by 1.5-5 times in the direction orthogonal to the initially stretched direction at a temperature of 80.degree.-150.degree. C., and heat-setting the thus treated film under tension at a temperature in the range from 180.degree. C. to the melting point of the polymer (Japanese Patent Application Laying Open (KOKAI) No. 55-111235 (1980)); a process for producing a poly-p-phenylene sulfide film, comprising melting a poly-p-phenylene (amorphous) sulfide containing not less than 90% by mole of recurring units: ##STR9## and having a melt viscosity of 100-600,000 poises as measured at 300.degree. C. and shear rate of 200 sec.sup.-1, extruding the melt onto a cooling medium having a surface temperature of 120.degree. C. or below to form a film having a density of 1.320-1.330 g/cc, uniaxially stretching the thus obtained film by 3-4.7 times at 85.degree.-100.degree. C., then further stretching the thus stretched film by 2.7-4.5 times in the orthogonal direction to the initially oriented direction at 87.degree.-110.degree. C. and heat-setting the thus treated film at 200.degree.-275.degree. C. (Japanese Patnet Application Laying Open (KOKAI) No. 56-62128 (1981)); a biaxially oriented poly-p-phenylene sulfide film having a gradient of 0.01-1.0 kg/mm.sup.2 /% at 20% elongation in the stress-strain curve when a 10 mm film piece cut out from said film in the longitudinal and transverse directions is stretched at a rate of 600%/min at 25.degree. C., produced by melt extruding and molding a poly-p-phenylene sulfide containing not less than 90% by mole of recurring units: ##STR10## and having a melt viscosity of 300-100,000 poises as measured at 300.degree. C. and shear rate of 200 sec.sup.-1 to form a substantially amorphous film, biaxially stretching this film simultaneously or successively at a temperature of 80.degree.-120.degree. C., and heat-setting the thus stretched film under tension at a temperature in the range from 180.degree. C. to the melting point of the polymer for 1-10 minutes (Japanese Patent Application Laying Open (KOKAI) No. 56-62127 (1981)); and a process for producing a poly-p-phenylene sulfide film comprising melt extruding and molding a poly-p-phenylene sulfide containing not less than 90% by mole of recurring units: ##STR11## to form a non-crystalline (amorphous) poly-p-phenylene sulfide film, biaxially stretching this film at 80.degree.-100.degree. C., heat-setting the thus stretched film at 150.degree.-280.degree. C., and subjecting the thus treated film to a heat treatment at a temperature in the range below the heat-setting temperature but above 50.degree. C. while shrinking or stretching within 20% in the longitudinal and transverse directions (Japanese Patent Publication No. 59-5099 (1981)).
Paraphenylene sulfide polymer, however, has the problem that its crystallization rate is too high in the melting work and it tends to form coarse spherulites. For instance, in case of forming a film by inflation method, the polymer is crystallized and hardened before a sufficient expansion occurs, and it is difficult to obtain a desired oriented film. Also, in case of extruding and molding the polymer into a sheet by a T-die, crystallization and hardening take place before the sheet is taken up on a take-up roll, and it is unable to obtain a flat and smooth sheet having a uniform thickness.
For overcoming these problems in working of paraphenylene sulfide polymer, there has been proposed an injection molding article, extrusion molding article or wire-coating article of a paraphenylene sulfide block copolymer essentially composed of recurring units (A): ##STR12## and recurring units (B): ##STR13## in which the recurring units A exist as a bonded block of average 20 to 5,000 units of the recurring unit (A) in the molecular chain and the molar fraction of recurring unit (A) is in the range of 0.50-0.98, said copolymer having a melt viscosity (.eta.*) of 50-100,000 poises as measured at 310.degree. C. and shear rate of 200 sec.sup.-1, a glass transition temperature (Tg) of 20.degree.-80.degree. C., a crystalline melting point (Tm) of 250.degree.-285.degree. C. and a crystallization index (Ci) of 15-45 (measured with the non-oriented heat-treated polymer film) (Japanese Patent Application Laying Open (KOKAI) No. 61-14228 (1986)).
The paraphenylene sulfide block copolymer which is essentially composed of recurring units (A): ##STR14## and recurring units (B): ##STR15## wherein the recurring units (A) exist as a bonded block of average 20-2,000 units of the recurring unit (A) in the molecular chain and the molar fraction of recurring units (A) is in the range of 0.50-0.98, and which has a melt viscosity (.eta.*) of 1,000-50,000 poises as measured at 310.degree. C. and shear rate of 200 sec.sup.-1, a glass transition temperature of 20.degree.-80.degree. C. and a crystalline melting point of 250.degree.-285.degree. C., has the same degree of crystallinity and heat resistance as paraphenylene sulfide homopolymer, is free of the problems in melt works of the homopolymer and also has a salient working characteristic that it can be well molded and worked even in a supercooling temperature range.
However, the biaxially oriented film of the paraphenylene sulfide block copolymer is not sufficiently high in Young's modulus, and thus a new proposal of a process for producing a paraphenylene sulfide block copolymer oriented film having a Young's modulus over 400 kg/mm.sup.2 has been desired.
As a result of studies for overcoming the abovementioned defects, the present inventors could solve the technical problems of too high crystallization rate and coarse spherulite forming tendency of paraphenylene sulfide homopolymer, and poor heat resistance of paraphenylene sulfide random copolymer by stretching the paraphenylene sulfide block copolymer film in a specific temperature range, and it has been found that it is possible to obtain a paraphenylene sulfide block copolymer stretched film having crystallinity and heat resistance of paraphenylene sulfide homopolymer, easy melt workability of paraphenylene sulfide random copolymer and a Young's modulus higher than 400 kg/mm.sup.2. The present invention was attained on the basis of this finding.